Method and apparatus for splicing compressed information streams

ABSTRACT

A method and apparatus for splicing a first compressed digital information stream into a second compressed digital information stream. The first information stream includes at least one entrance indicium identifying an appropriate stream entrance point, the second information stream includes at least one exit indicium identifying an appropriate stream exit point. A controller monitors the two streams until the appropriate points are found and, in response to a control signal, splices the first stream into the second stream.

This invention was made with U.S. government support under contractnumber 70NANB5H1174. The U.S. Government has certain rights in thisinvention.

This application claims the benefit of U.S. Provisional Application No.60/018,554, filed May 29, 1996.

This application is related to U.S. patent application Ser. No.08/864,321 (Attorney Docket No. 12070), filed on the same date as thepresent application.

The invention relates to communication systems in general, and moreparticularly, the invention relates to a method and apparatus forsynchronizing a plurality of compressed data streams to facilitatestream selection, splicing and other operations.

BACKGROUND OF THE DISCLOSURE

In several communications systems, the data to be transmitted iscompressed so that the available bandwidth is used more efficiently. Forexample, the Moving Pictures Experts Group (MPEG) has promulgatedseveral standards relating to digital data delivery systems. The first,known as MPEG-1 refers to ISO/IEC standards 11172, incorporated hereinby reference. The second, known as MPEG-2, refers to ISO/IEC standards13818, incorporated herein by reference. A compressed digital videosystem is described in the Advanced Television Systems Committee (ATSC)digital television standard document A/53, incorporated herein byreference.

A program transport stream is formed by multiplexing individualelementary streams which share a common time base (i.e., the same 27 MHzclock source). The elementary streams comprise encoded video, audio orother bit streams. The elementary streams may be, but do not have to be,in a packetized elementary stream (PES) format prior to transportmultiplexing. A PES consists of a packet header followed by a packetpayload. As the elementary streams are multiplexed, they are formed intotransport packets and a control bit stream that describes the program(also formed into transport packets) is added.

There are many instances where there is a need to switch from oneencoded or compressed bitstream to another. When switching from onecompressed MPEG video bitstream to another, appropriate measures must betaken in the transmission order of the picture bitstream to assureproper subsequent presentation of the decoded pictures, without timegaps. Such time gaps result in undesirable video or audio artifacts(e.g., blank screen due to buffer overflow/underflow, poor "lip sync"and the like). Heretofore there has not existed a seamless splicingmethod and apparatus for splicing transport streams to one another.

Therefore, a need exists in the art for a method and apparatus forsplicing compressed digital information bitstreams.

SUMMARY OF THE INVENTION

The disadvantages heretofore associated with the prior art are overcomeby the present invention of a method and apparatus for splicingcompressed digital information streams. In particular, the inventionsplices a first information stream into a second information stream. Thefirst information stream includes at least one entrance indicium thatidentifies an appropriate point of entrance to the stream. The secondinformation stream includes at least one exit indicium that identifiesan appropriate point of exit from the stream. The invention monitors thetwo streams until the appropriate points are found and, in response to acontrol signal, splices the first stream into the second stream.

Specifically, the inventive splicer includes a pre-splice bufferreceiving a first information stream and producing a bufferedinformation stream; a bitstream examiner receiving the first informationstream and responsively causing the pre-splice buffer to position anentrance point of the buffered information stream at an output of thebuffer; a switch for coupling either the buffered information stream ora second information stream to an output; and a switch controller formonitoring the second information stream and, in response to a controlsignal and the detection of an exit point in the second informationstream, causing the switch to couple the buffered information stream toan output.

BRIEF DESCRIPTION OF THE DRAWINGS

The teachings of the present invention can be readily understood byconsidering the following detailed description in conjunction with theaccompanying drawings, in which:

FIG. 1 shows a block diagram of a compressed bitstream splicing systemincluding the invention;

FIG. 2 depicts a flow chart of a seamless splicing process in accordancewith the invention;

FIG. 3 shows a detailed block diagram of the splicer of FIG. 1;

FIG. 4 depicts a block diagram of digital studio comprising a pluralityof interoperable islands and including the invention; and

FIGS. 5A-5C depicts a plurality of splicing scenarios.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

The invention is generally described within the context of a digitaltelevision studio includes a plurality of operative environments whichreceive and process various bitstreams and which have associatedswitching capabilities according to the invention. The switchingcapabilities allow seamless or non-seamless splicing of a plurality of,e.g., video transport streams to produce an output stream. A combinationof seamless and non-seamless bitstreams may be produced to provide acontrollably degraded output stream.

The invention is a two-input bitstream splicer which performs switching,splicing or insertion operations on a pair of MPEG-compliant inputtransport streams to produce an output stream. It must be noted that theprinciples of the invention apply to bitstream switchers or splicershaving more than two inputs and to input streams other thanMPEG-compliant input streams. The invention may be implemented using ageneral purpose computer system that is programmed to perform thefunctions discussed below. As programmed, the general purpose computerbecomes a specific purpose apparatus for splicing digital data bitstreams.

The invention may be used for both seamless and non-seamless splicing ofbitstreams. Seamless splicing means seamless butt-splicing of twostreams to form a resultant output stream that produces a continuous,undisturbed flow of information (e.g., video or audio without glitchesor artifacts). Non-seamless splicing produces a resultant output signalwhich may have a disturbed information flow (e.g., visual or auraldistortions, disturbances and artifacts). For purposes of thisdiscussion, it will be assumed that each bitstream is a transport streamcomprising video, audio and (possibly) other information. It must benoted that the invention is applicable to packetized elementary andother elementary streams. Additionally, it is assumed that the splicingpoints are determined with respect to the video information. This mayresult in some distortions in the spliced audio and other information,since the audio and other information may not temporally "line up" on apacket by packet basis.

Splicing consists of making a transition in an output-stream from a"from-stream" to a "to-stream." The from-stream is ideally exited at an"out-point" and the to-stream is ideally entered at an "in-point." Anout-point is a place in a presently-selected stream (i.e.,"from-stream") where the stream may be ended, and some other stream(i.e., "to-stream") spliced on. An "in-point" is a place in the otherstream where the information may begin to be spliced on to anotherstream.

A "splicing segment" is defined as the portion of an information streambetween an in-point and an out-point. A splicing segment may includemultiple out-points and in-points. Thus, it is desirable to include asmany in-points and out-points as possible in a stream to allow formaximum flexibility in splicing. Within the definition of an in-pointand an out-point is a delay-parameter, e.g., a video buffering verifier(VBV) for MPEG compliant streams. A splicing segment with a knownin-point delay-parameter and with out-points having the same knowndelay-parameter may include within itself shorter valid splicingsegments with different values of the delay-parameter.

In the context of a studio environment of an exemplary embodiment of theinvention, information streams are divided into transport packets.Packets containing video may be intermixed with packets containingaudio, auxiliary data, or other information. In this environment, avideo stream out-point is the end of the last video transport packet ofthe stream of interest. The video stream before and through the lastpacket must meet the splicing definition of an out-point. Similarly, avideo stream in-point is the beginning of the first video transportpacket of a splice segment (SS). It must be noted that other informationin the transport stream, specifically audio, is unlikely to be neatlysegmented at in-points and out-points. A method for correcting errorsinduced by the non-alignment of audio transport packets is described inU.S. patent application Ser. No. 08/864,321, filed simultaneouslyherewith (Attorney Docket 12070), incorporated herein by reference.

A critical aspect of splicing information streams is the properprocessing of the various delay parameters. One parameter of concern isthe delay parameter associated with the various information streams. Inthe case of an MPEG-compliant stream, the delay parameter is the videobuffering verifier (VBV) delay parameter. Another parameter is thelatency, or transitional period, inherent in a splicing operation. Forexample, a typical splice occurs at a certain time, i.e., a "splicetime." Prior to the splice time an output information stream comprises afrom-stream. At the splice time, a switch to the to-stream occurs. For aperiod of time the output stream may include information from both thefrom-stream and the to-stream. Eventually the output stream includesinformation from only the to-stream.

It is assumed that the from-stream and the to-stream are each valid.There are certain constraints on the streams that must be met if thesplicing is to be seamless. Seamless splicing implies that the resultantspliced bitstream will not cause discontinuities in the future.

One specific example of a valid splice segment that can be seamlesslyspliced is an MPEG-compliant splice segment. An MPEG Splice Segment (SS)is defined at the transport level and includes functionality at thevideo (and audio) levels. An information-bearing splice segment may beas short as a single frame. A splice segment may even be a zero framelength segment (although such a SS might be MPEG non-compliant). Such azero-length segment is simply an in-point followed by an out-point(i.e., an "in-out-point"). A SS may be also be very long, including manyGOPs. In general the length of a SS is not constrained and the SS shouldinclude multiple out-points to enable seamless exiting from the segment.A possible exception is a SS comprising a television commercial. Thetelevision commercial SS can be deliberately produced without definedout-points so that exiting the commercial segment is not seamless.

An MPEG SS should be an MPEG compliant stream having consistenttransport stream and elementary stream time stamps (e.g., PCR, PTS andDTS) and an associated delay parameter (e.g., a VBV delay), therebyallowing a decoder to properly decode and present the information in theSS. The first information frame (e.g., video access unit) at an in-pointof an MPEG video SS must be an I-Frame. The second frame shall notreference information frames prior to the in-point (i.e., if the secondframe is a B-frame, the B-frame may not reference frames prior to thein-point). The last frame before an out-point should not be a B-frame(in display order). An audio SS will have an in-point consisting of thebeginning of an audio frame and an out-point consisting of the last byteof an audio frame. There may be other constraints placed on the streamto address issues of, e.g., coding error-build-up, tuning-time andminimum picture quality.

The in-point of a video SS must begin with a sequence header, althoughthe SS may contain multiple sequence headers. A SS may containadditional header information to indicate that the sequence header isalso an in-point. It is necessary to distinguish the SS in-pointsequence header from a sequence header included for tuning-time orpicture quality, since seamless splicing can only be guaranteed onin-points. Since the in-point should follow an sequence end code (SEC)code it is desirable to include the SEC code just before the in-point,thereby obviating the need to include the SEC on the end of anout-point. The out-point may include the SEC. An MPEG-type splicecount-down, if used, must end (i.e., equal zero) at the out-point.

FIG. 1 shows a block diagram of a compressed bitstream splicing system100 including the invention. The system 100 includes a first compressedbitstream stream source 110, a second compressed bitstream stream source120, a splicer 300, a controller 105 and an optional splice monitor 130.The first compressed bitstream stream source 110, illustratively a "livefeed" from a transport stream encoder, produces a first MPEG-complianttransport stream S6. The second compressed bitstream stream source 120,illustratively a server (e.g., a video disk, tape machine, or otherstorage device) which stores video and audio elementary streams andtransport encodes the stored streams to produce a second MPEG-complianttransport stream S7. The stored information may comprise, e.g.,advertisement or local programming information to be spliced into thefirst transport stream. The splicer 300 selectively couples one of thetwo input transport streams S6, S7 to a transmitter or other subsystemas an output stream S9. An optional splice monitor 130 monitors variousparameters of the spliced output signal S9, e.g., delay parameter,buffer utilization information, synchronization, bitstream source andthe like. The optional splice monitor 130 is responsive to thecontroller 105 and the splicer 300.

The splicer 300 receives the first transport stream S6, illustratively atelevision program produced by a first source, and the second transportstream S7, illustratively an advertisement produced by a second source.In response to a control signal SELECT, the splicer produces an outputsignal S9 comprising either the first S6 or second S7 transport stream.The control signal SELECT may include priority information which causesthe splicer 300 to respond immediately, within a defined time intervalor when certain conditions exist (i.e., specific alignments of streamentrance or exit points). The splicer 300 produces a signal ACKNOWLEDGEwhich is used to acknowledge the SELECT signal and provide specificdetails about the splice operation (e.g., exact time of splice, errorconditions and the like). The operation of the splicer 300 is describedmore fully below with respect to FIG. 3.

The actual splicing operation is the process that takes place within thesplicer 300 that does what is necessary to actually switch amongst thebitstreams. This involves stopping, in an orderly manner, the flow ofpackets from the from-stream; starting, in an orderly manner, the flowof packets from the to-stream; and adjusting the header information inthe output stream. During some interval, packets from both thefrom-stream and the to-stream are likely to be intermixed.

Splicing operations must be synchronized to be seamless. To ensure thatinput streams arrive at the appropriate splicers at the time they areneeded several synchronizing operations may be performed. It is assumedthat the output stream is continuous and that the actual splice is takento be a change in the content of the output stream from a from-stream toa to-stream. The time stamps in the output stream should also maintaincontinuity from one stamp to the next (this is related to streamcontent) and the splicing mechanism should adjust the output streamtime-stamps. In the absence of time stamp continuity in an MPEG system,the MPEG "discontinuity" header flag should utilized such that anindication of a new time stamps (or time stamp discontinuity) isprovided to a decoder.

To accomplish the adjustment the splicing process must have some notionof time, since this local notion of time that must be used to producethe output time-stamps. The splicing process gets its notion of timefrom some timing source such as the OC-12c interface and the currenttime is derived from either stream content or set-time messages. Thelocal notion of time must be moderately continuous and well behaved.When splicing, both the end of the from-stream and the beginning of theto-stream must be available at the actual splice hardware that isproducing the output. In addition, all buffering within the splicingprocess must be finite and defined.

In addition to the above issues, there are synchronization issues to beconsidered. For example, it is important to consider the effect ofpacket jitter on the splicing process. If any additional information isrequired, beyond that contained within the actual streams being spliced(e.g. priority information, source identification, error codes and thelike), the additional information must be properly synchronized with theactual splice streams.

There are several conditions that are of interest with respect tosynchronization of the splicing function. These are the timingrelationships between desired operation and actual operation,continuous-flow streams, server-generated streams and remotely-generatedstreams.

The timing relationships between desired operation and actual operationwill be discussed first. At some operational unit, e.g. a Play-to-AirSwitcher, a decision to switch streams must be made. The source of anoutput stream is actually switched in response to that decision.

The decision to splice may be content related, such as a switch from afrom-stream to a to-stream when a content-related data element isencountered in one of the streams. For example, the from-stream may bemonitored and, in response to the detection of, e.g., a black-screen ora scene change, a splice decision may be made. This operational decisiondoes not require synchronization. Rather, the decision requires that thesplicer (or a controller) analyze, e.g., the from-stream to detect thedata element. The decision to splice may also be data-flow related, suchas a switch from a from-stream to a to-stream on some particular packetor upon the start or stop of information flow.

The decision to splice may be time-related, such as a switch from aprogram to commercial at noon. Time-related decisions must be referencedto the splicer's local frame-of-reference. A message-passing processpasses the decision information to the splicer in time for the splicerto be ready to make the splice in its frame-of-reference. Given that thedecision to splice at some time has been made, the splice will be madeat the next available splice point, based upon the from-stream and theto-stream.

The decision to splice may be may be event driven, such as the pushingof a button (e.g., the director's "take" command, as depicted in thesplicer 100 of FIG. 1). When the message indicating the event arrives atthe splicer, the actions are the same as those for a time-relateddecision whose time has arrived.

Some form of acknowledge message may be required. This message, whendelivered to the originator of the splice decision (e.g., thecontroller), will allow an intelligent choice to be made abouttime-outs, and actions like panic non-seamless splices. Time-outs anddeterminations about corrective actions to remedy splice failures is apolicy matter for the originator of the splice decision. Time-out andforced switch may be a service implemented by the splicer but only as aconvenience.

An operational unit (e.g., splicer or switcher) may feed back anappropriate acknowledgment message to a controlling entity. The contentsof such a feedback message may include one or more of the followingparameters: 1) a splice did or did not take place; 2) the localtime-of-day that the splice occurred; 3) the delay-parameter value ofthe to-stream; 4) the delay-parameter value of the from-stream; 5) thecurrent (post-splice) sync-buffer delay (e.g., in delay seconds); 6) thefuture time a splice will take place (if the switcher can compute thisvalue); and 7) any exceptions or errors. Exceptions and errors mayinclude the fact that no splice took place, that the decision parameterspassed by the controller were incorrect (e.g., syntactically orlogically), that the to-stream was not ready, that a time-out occurredor that an audio-failure occurred (e.g., the dropping of an excessivenumber of audio frames).

Additional information that may be of value includes: 1) the amount oftime that the audio information from the from-stream will be needed; 2)an indication that the inputs are buffered correctly and ready for a newsplice; and 3) other information useful to the controller or thesplicing process itself.

The precise time at which a seamless splice takes place may not bepre-determined, since the seamless splice depends upon the arrival of anin-point in the to-stream. In the case of a decision to spliceseamlessly there are several sub-decisions which must be made about whatto do if the splice does not take place within some time limit. Thechoices are as follows. First, simply wait for a seamless splice tooccur. Depending upon studio operational goals, this may not beacceptable. Second, define a fixed time-out period and, if the splicerhas not spliced within the defined time-out period, perform anon-seamless splice (i.e., switch streams in as controllable a manner aspossible). While this is a simple approach, it limits decisionflexibility. Third, simply wait for the splice to occur but acknowledgethis condition to the controlling entity. Whatever process generated thesplice decision may then decide to allow a non-seamless splice. Thisallows reasonable operational flexibility. Fourth, provide aprogrammable time-out, optionally including a default time-out andoptionally including an acknowledgment of this condition to thecontrolling entity. This option allows the controlling entity to alterthe time-out value, thereby providing the greatest operationalflexibility. It should be noted that a time-out is a secondaryoperational feature and, therefore, an exception to normal operation.

The synchronization of continuous-flow stream splicing will now bediscussed. The streams arrive and must be buffered (e.g., one second'sworth of information) within the splicer in some manner so that, e.g.,to-stream in-points are available when out-points occur in thefrom-stream. If the amount buffered is insufficient (e.g., more than asecond elapses between successive in-points in a from-stream), then thebuffer will overflow and will contain invalid information. Thiscondition is remedied by an appropriate number of in-points andout-points being inserted into the bitstreams. If bitstreams do not havein-points and out-points often enough, then those bitstreams can not beseamlessly spliced at those times. Moreover, to the extent that there ispacket or cell jitter in the arrival time of input bitstreams, afirst-in, first-out (FIFO) buffer (with output clocked at nominal datarate) is expected to smooth the flow.

The synchronization of server generated streams will now be discussed.Server-generated streams must be carefully generated so that the datadoes not arrive at the splicer too early or too late. If the dataarrives too early, there is some risk of overflow of an input buffer. Ifit is assumed that the splicer has enough synchronization buffering tohold a second or so of video, then it would seem that server streams canbe delivered in any pattern of flow that never exceeds the just-in-timelimit, and the one-second-early limit. Of course, there may be peak rateimitations on the splicer.

The synchronization of remotely-generated streams will now be discussed.It must be noted that any stream processed in a studio containing thesplicer is expected to have the same reference clock rate.Remotely-generated streams, by the time they have reached a splicer,should be the same as locally-generated real-time streams. To referenceremotely-generated streams to a local master clock the remote source maybe genlocked to the local studio. This can be done via a reverse channelor by locking both to an external reference, such as a timing signalderived form the Global Positioning System (GPS). If there are twoindependent studios, each with an independent master clock, and each isdoing a remote feed to the other, then one will be delivering data tooslow for the other, and one will be delivering data too fast. Anothermethod is to delay a remote feed by a time equal to the maximum clockdrift over some operating interval. A 30 ppm. drift rate, over 24 hours,accumulates 2.6 seconds. A six second buffer, initialized to a 3 secondfill, is adequate to absorb clock drift.

Splice monitoring is an important aspect of splicing, especially in astudio environment. Content-related monitoring may comprise the steps ofviewing an image on a display device (i.e., "monitor") responsivelychanging parameters of the bitstreams producing the image (e.g.,splicing). Optional splice monitor 130 may be used for content-relatedmonitoring by, e.g., a director. Another form of monitoring is thequalitative assessment of a monitored bitstream. Optional splice monitor130 may be used to retrieve qualitative information from the splicedoutput signal S9, e.g., delay parameter, buffer utilization information,synchronization information, bitstream source identification and thelike. The optional splice monitor 130 is responsive to the controller105 and the splicer 300 to either process the information and return,e.g., an operational summary, or to couple the qualitative informationdirectly to the controller 105 and the splicer 300 for furtherprocessing.

With respect to content-based monitoring, a director (i.e., human) maybe viewing the various bitstreams via monitors (i.e., decoders drivingdisplays) and responsively splicing to produced an output stream (i.e.,a program). This issue is especially significant in production or liveswitching with complex-GOP streams. The problems are greatly minimizedif operating in a low-delay (e.g., I-frame only) production mode. Whenthe director indicates that a splice is to be made "now," that decisionis likely to be based upon what the director sees and expects to see. Asa decoder inherently presents its picture after some delay from itsinput bitstream (e.g., part or all of end-to-end delay and someadditional decode and format delay), the modes of operation for thedirector in an MPEG studio environment will be different from currentNTSC-related operations. For purposes of the following examples, assumethat a director is operating in an MPEG studio environment, that splicesegments are 1/4 second long, and that the end-to-end delay of thespliced stream is 1/2 second.

A first example is a "soonest" mode of operation. In this mode, thedirector presses a "take" button TAKE based upon an event seen on anoutput monitor 132, a from-stream monitor 136 or a to-stream monitor134. A queued up (e.g., server-stored) to-stream is ready and aligned atan in-point. Within 1/4 second of the splice command an out-point willarrive at end of the from-stream sync-buffer and the splice will bemade. It must be noted that the from-stream contains up to 1/4 second ofdelay. One monitor delay later (1/2 second) the scene on the outputmonitor changes.

If the director responded to a scene on the output monitor 132, theamount of output monitor delay (i.e., the time between the "take"command TAKE and a change in scene on the output monitor 132) is between1/2 and one second. If the director responded to a scene on thefrom-stream monitor 136 , the amount of from-stream monitor delay isbetween 1/4 and 1/2 second and the output monitor delay is 1/2 second.If the director responded to a scene on the to-stream monitor 134, theto-stream monitor 134 is continuous (i.e., no monitor delay) and theoutput monitor delay is negative 1/4 seconds (i.e., the scene changes1/4 second after the "take" button TAKE is pressed and the imagedisplayed occurred 1/4 second prior to the press of the button).

A second example is the "next" mode of operation. In this mode, a queuedup to-stream is flushed from a to-stream synchronization buffer and thenext segment beginning with an in-point is queued up within up to 1/4second. The to-stream synchronization buffer also has zero to 1/4seconds of random delay. When the in-point arrives the splice is made.

If the director responded to a scene on the output monitor 132, theamount of output monitor delay is between 1/2 and one second. If thedirector responded to a scene on the from-stream monitor 136, the amountof from-stream monitor delay is between 1/2 and 3/4 second and theoutput monitor delay is 1/2 second. If the director responded to a sceneon the to-stream monitor 134, the to-stream monitor 134 is continuousand the output monitor 132 switches to a new scene between zero and 1/4second later.

The choice of "soonest" or "next" mode of splicing is an operationalone, and may be based upon which disconcerting effect (delay or back-up)is least objectionable. To alleviate these effects an amount of delaymay be inserted into the splicer inputs. If this delay matches themonitor delay, and the monitors are connected to the inputs of thedelays, then the apparent delay between monitor scenes and button actionis less, but the delay to final output is greater. In addition, aseparate monitor control unit may be built to simulate the bit-streamswitching and show the simulated results of the bitstream switch,thereby providing more flexibility to the director.

FIG. 3. shows a detailed block diagram of the splicer 300 of FIG. 1. Thesplicer 300 selects one of a first input bitstream 86 and a second inputbitstream 87 as an output bitstream S8. The output bitstream 88 isoptionally time stamped to produce a retimed output stream S9. The firstand second input bitstreams S6, 87 are, illustratively, MPEG-complianttransport streams including at least video and audio elementary streams.The video and audio elementary streams may be in a packetized elementarystream (PES) format.

For purposes of the following discussion it is assumed that the secondbitstream S7 is currently selected as the output bitstream (i.e., S7 isthe from-stream) and the first bitstream S6 will be selected as theoutput bitstream (i.e., S6 is the to-stream) after a splicing operation.

The first input bitstream S6 is coupled to a first bitstream examiner310A and a first synchronization buffer 320A. The first bitstreamexaminer 310A examines the first bitstream for entrance points whichhave been included in the first input bitstream S6. When an in-point isfound, the contents of the synchronization buffer are discarded (i.e.,the buffer is "flushed") and the in-point is stored in the first memoryportion of the synchronization buffer. The synchronization buffer may beconstructed as a first-in, first-out (FIFO) buffer. The process ofsearching for in-points and flushing the buffer is repeated until thefirst input bitstream S6 is selected by the splicer. In this manner,there is always an in-point positioned at the end of the synchronizationbuffer of the stream that is not being currently output. The outputbitstream S3A of the first synchronization buffer 320A is coupled to aswitch controller 340 and a first working buffer 330A. The first workingbuffer 330A produces an output signal S4A which is coupled to a packetswitching unit 350.

The second input bitstream S7 is coupled to a second bitstream examiner310B and a second synchronization buffer 320B. If the second bitstreamwere not presently selected as the output stream, then the secondbitstream examiner 310B and synchronization buffer 320B would operate inthe same manner as described above with respect to the first bitstreamexaminer 310A and synchronization buffer 320A. The second bitstreamexaminer 310A examines the second bitstream for exit points which havebeen included in the second input bitstream S6. In the "selected mode"of operation, the second bitstream examiner 310B is not used and thesecond synchronization buffer 320B serves as a constant delay bufferwhich produces a delayed bitstream S3B.

The delayed bitstream S3B is coupled to a working buffer 330B and aswitch controller 340. The second working buffer 330B produces an outputsignal S4B which is coupled to packet switching unit 350. The secondworking buffer 330B holds the selected bitstream long enough to allowfor overlap of old audio packets with current video packets. This allowsaudio frames to continue to completion after a splice is made. Thesynchronization of audio and video frames are discussed in more detailbelow and in U.S. patent application Ser. No. 08/864,321, filedsimultaneously herewith (Attorney Docket 12070) and incorporated hereinby reference.

A splice decision is made by a controller (e.g., controller 105) andcoupled to the switch controller 340 via a control signal SELECT.Assuming that the splice decision equates to the command "spliceseamlessly at the next opportunity," the switch controller 340 respondsby scanning the currently selected output stream (i.e., bitstream S3B)for out-points. It is assumed that an in-point is positioned at the endof the first synchronization buffer 320A. When an out-point arrives onthe from-stream, the switch controller 340 causes, via a control signalA/B, the switch 350 to begin coupling video packets from the to-streamthrough the switch to an optional header adjuster. At an appropriatetime any audio packets within the to-stream are also switched.

The optional header adjuster 360 alters time-stamps in the selectedoutput stream S8 to produce a retimed output stream S9. The retiming ofthe program clock reference (PCR), presentation time stamps (PTS) anddecode time stamps (DTS) of the selected stream S8 may be necessary toensure that the splice is, in fact, seamless to a decoder. The headeradjuster 360 includes a 27 MHz (local) station clock 362 which isutilized by a local PCR and PCRB generator 364. To retime thepresentation and decode time stamps it is necessary to partially decode(i.e., packetized elementary stream (PES) layer) the selected transportstream S8. The partial decoding and retiming of the PTS and DTS isperformed by a PTS and DTS detection and retiming unit 366 to produce aPTS and DTS retimed stream S8P. The PTS and DTS retimed stream istransport encoded and time stamped by PCR detection and retiming unit368 to produce a retimed transport stream S9. An alternate embodiment ofa header adjuster is the PTS-DTS retimer discussed in more detail inU.S. patent application Ser. No. 08/864,326, filed simultaneouslyherewith (Attorney Docket 12389) and incorporated herein by reference.

As previously mentioned, the invention may be implemented using ageneral purpose computer system that is programmed to perform thevarious functions. The embodiment of FIG. 3 may be implemented as acomputer program utilizing portions of memory to provide buffering, andan algorithm directed to the examination, control, switching and headeradjustment functions. As previously mentioned, the splicer 300 producesa signal ACKNOWLEDGE which is used to acknowledge the SELECT signal andprovide specific details about the splice operation (e.g., exact time ofsplice, error conditions and the like). A routine for splicing will nowbe described with respect to FIG. 2.

FIG. 2 illustrates a splicing routine in accordance with the invention.The splicing routine is entered at step 202 when the decision to spliceis made. For the purpose of this discussion, it is assumed that thedecision is to seamlessly splice from the currently selected (from)stream S4B to another (to-stream) stream S4A. The decision is examinedat step 204. If the decision of step 202 is to splice as soon aspossible, then the routine proceeds to step 208. If the decision is tosplice at the next in-point (e.g., skip the presently buffered GOP inthe to-stream), then the synchronization buffer (e.g., 320A) is flushed.When the to-stream synchronization buffer (e.g., 320A) has a validin-point stored (step 208) and the from-stream is at a valid out-point(step 210), the splice is made (step 220) and the routine is exited(step 230). The context in which a splicing decision (step 202) is madeis relevant to the amount of information necessary to perform a seamlesssplice. If the splice decision is made in the context of buildingplay-to-air edit lists, it is necessary for the streams to be spliced tohave the same value of delay-parameter. If the splice decision is madein the context of creating a live production, it is necessary for thestreams being spliced to have matching delay-parameters and splicepoints which occur often enough to meet operational requirements. If thesplice decision is made in the context of creating a live production,and the production involves information stored on servers, it is helpfulto know that anticipated splice-points are about to arrive soon (via,e.g., splice point countdowns or splicing tables). The splicing decisionand related matters will be discussed in detail below.

The invention will now be described within the context of a digitaltelevision studio including a number of distinct operating environments(such as servers or edit-suites) which receive, process and transmitvarious information streams. The operating environments, or "islands ofinteroperability," may be interconnected to perform one or moreoperations on the various information streams. The studio output may bedelivered to end-users (e.g., the public) via ATSC broadcast, cable,telephone and satellite transmission and the like. The studio output mayalso be stored for later use in, e.g., a server or on CD-ROM or videotape. The invention is also useful in video teleconferencing and otherapplications.

While the streams delivered to broadcast customers must meet, e.g., ATSCstandards, it is not necessary to deliver all of the internal studioinformation. For example, high bit-rate studio formats are useful onlywithin the studio or studio-like environments. When dealing withsplicing, there may be information within the stream that is meaninglessto consumer decoders, but which is necessary to studio splicing.

FIG. 4 depicts a block diagram of digital studio comprising a pluralityof interoperable islands and including the invention. The digital studio400 of FIG. 4 includes interoperable islands 401, 402 and 404-409. Thedigital studio 400 also includes a first compressed bitstream streamsource 110, a second compressed bitstream stream source 120, a splicer300, a controller 105 and an optional splice monitoring unit. The firstcompressed bitstream stream source 110, illustratively a "live feed"from a transport stream encoder, produces a first MPEG-complianttransport stream S6. The second compressed bitstream stream source 120,illustratively a server (e.g., a video disk, tape machine, or otherstorage device) which stores video and audio elementary streams andtransport encodes the stores streams to produce a second MPEG-complianttransport stream S7. The first and second compressed bitstream sources110, 120 operate in substantially the same manner as previouslydescribed with respect to the bitstream splicing system 100 of FIG. 1.The digital studio 400 includes a controller 105 which performs thosefunctions previously described with respect to the bitstream splicingsystem 100 of FIG. 1, and other functions which will be described below.Island 300 roughly equates to the splicer 300 of the bitstream splicingsystem 100 of FIG. 1.

Each of the islands receives a plurality of information streams. Forexample, islands 401 and 402 each receive information streams from aNETWORK FEED and a LOCAL FEED. Controller 105 communicates with each ofthe islands via a control channel C. The control channel C is used todirect the flow of information throughout the studio (i.e., betweenislands) and to direct the processing of the information within theislands. The controller 105 provides the splicing decisions and anynecessary parameters associated with the intended splice. The islandsrespond by performing, monitoring and acknowledging (via control channelC) the various splicing operations.

A digital studio according to the invention may be described as aninterconnected group of "splicing islands" which perform particularprocessing functions on received bitstreams to produce outputbitstreams. This is because the islands form individually distinctoperating environments (e.g., storage environments, editingenvironments, processing environments and the like) which cooperate witheach other via a controller 105 to produce one or more output bitstreams(e.g., S9, OUTPUT STREAM). Each island operates at a knowndelay-parameter value and all splicing within an island is (ideally)seamless. The splicing and processing functions are under the generalcontrol of controller 105, but may be locally controlled as necessary.For example, an operator sitting at an editing station may logicallycomprise one island. The streams to be edited are routed to an editingisland (e.g., island 407) in response to commands transmitted viacontrol channel C from controller 105. In response to the commands asignal may be switched through several islands (e.g., 406 and 300) priorto being stored in a storage unit (not shown) at the editing island(e.g., 407).

An alternate mode of studio operation is to controllably operate one ormore islands in a non-seamless mode. The non-seamless mode may berequired in several circumstances where a splice or other transitionbetween bitstreams must occur rapidly, and a range of bitstreamdegradation is permissible. It must be noted that non-seamless switchingmay produce errors which are propagated to subsequent islands receivinga degraded bitstream. These errors may be mitigated, if necessary, by,e.g., dropping damaged or inferior access units or groups of accessunits (e.g., video frames) or by adding additional access units. Forexample, if a to-stream having a short delay-parameter is to be splicedonto a from-stream with a long delay-parameter, the splicing operationis unlikely to be seamless (i.e., the buffer will likely overflow). Inthis case, frames may be dropped to avoid the overflow condition. Also,when a to-stream with a long delay-parameter is to be spliced onto afrom-stream with a short delay-parameter, the splicer needs to adjusttime stamps to cause a number of frame repeats (i.e., add frames) whilethe buffer fills. The buffer may also be increased by splicing short,all-black frames on the end of a short delay-parameter sequence to buildup the value of the delay-parameter in current use.

In the exemplary embodiments of FIGS. 1, 3 and 4, splicing operationstake place in operational units (e.g., splicing islands), such asrouting switchers, play-to-air switchers, production switchers or otherswitchers. Therefore, it is desirable to support a plurality of dataformats and bitrates. For example, the so-called 422@HIGH and 420@HIGHtelevision studio formats each support multiple picture formats and bitrates. Therefore, it may be necessary to splice, e.g., a bitstreamcomprising a 1280 by 960 picture element, 60 Hz Progressive Scan pictureonto the end of a bitstream comprising a 1920 by 1080 picture element,59.94 Hz interlaced picture. Moreover, it may also be necessary tosplice a 45 Mb/s stream onto the end of a 155 Mb/s stream.

Both of the above example splices may be seamlessly made if the streamsbeing spliced have matching delay parameters. Therefore, it is importantthat the controller that makes the splice decision know the delayparameters of the various streams to be spliced. The delay parameter ofa stream may be calculated by an operational unit receiving a stream orincluded within the stream as part of the stream's header information.In the splicer 300 of FIG. 3, switch controller 340 includes a bitstreamcalculator which calculates the delay parameters of the input streamsS6, S7. Is should be noted that the delay parameter calculation may alsobe performed by the bitstream examiners 310A, 310B or the optionalsplice monitor 130.

Another critical aspect of splicing information streams is thedetermination of in-point and out-point locations in the streams to bespliced. To properly perform a seamless splice it is necessary to findthe in-point of the to-stream and the out-point of the from-stream.Moreover, a splice segment may include in-points and out-points havingdifferent delay-parameter values. There are several options availablefor finding the appropriate splice points.

First, the entire to-stream or from-stream may be analyzed by thesplicer in real time (i.e., "on the fly"). A real-time analysis isdifficult for a to-stream because an in-point cannot readily be deducedfrom the stream without playing the stream to its end. Even if it wereknown that the stream is well-behaved, the length of an I-frame is notknown in advance. By the time the first I-frame has ended, and itslength is known, it is probably too late for the information to be used.It must be noted that this problem may be overcome by using, e.g., amore powerful computing device. A real-time analysis is easier for afrom-stream because the delay-parameter of the from-stream is known(from the in-point or otherwise), the presentation time-stamps in thestream indicate when frames leave the decode buffer, and bit-counts (orpacket counts) indicate when the frames enter the decode buffer. Theframe rate is also known from sequence headers. With the foregoinginformation it becomes possible to calculate, at the start of a newframe, that the preceding video has left the buffer in an out-pointstatus. This information arrives only a little bit late.

Second, an external table may be created to contain indications of wheresplice points are. This approach assumes that the information aboutin-point and out-point locations has been computed elsewhere (e.g.,during a stream encoding process). This approach requires that thein-points and out-points be indexed in some manner (e.g., Nth packetfrom a marker, first packet after a time-of-day reference, and thelike). This approach also requires the updating of a splice tableassociated with an information stream as the information stream isprocessed, and the transmission or regeneration of the table as theinformation stream is transmitted (e.g., via satellite link). It shouldbe noted that the use of separate information tables to determine splicelocations is practical within a server or island of operability, thoughthis use of tables may be less practical when transmission encoding anddecoding are to be performed on the stream.

Third, in-point and out-point markers may be placed within theinformation stream directly. An MPEG compliant information streamincludes header portions where such a marker may be included. There areheader portions suitable for in-point and out-point marker insertion atthe system level, transport level and PES level. There are alsoopportunities to insert markers in the elementary streams.

Both in-points and out-point should be marked and, ideally, the markingshould occur at the system, transport and PES levels. In addition to theinsertion of in-point and out-point markers, the delay-parameterassociated with the stream or splicing segment and an audio offset(i.e., a displacement of audio-frame boundaries from associated videoframes) should also be inserted into one or more layers of theinformation stream. The MPEG count-down feature should also be used toindicate that, e.g., an out-point is approaching (decreasingly positivecountdown) or an in-point has been transmitted (increasinglynegative-countdown). For various business reasons it may be desirable toremove these markers prior to transmitting an MPEG or ATSC signal to anend-user (i.e., consumer). While end-users might want to splice thevideo, it is important that they not be able to clip out commercialsautomatically.

The above-described redundancy of marking provides a maximum flexibilityto system designers and provides a redundancy of operation which helpsto ensure that splicing operations are, in fact, seamlessly made (i.e.,made at appropriate in-points and out-points).

Bitstream Generation

To help ensure seamless splicing it may be necessary to create thebitstreams to be spliced in a certain manner. There are two facets tothe creation of bitstreams that can be spliced; the creation of thestream content, and the insertion of appropriate splice controlinformation (i.e., in-point and out-point markers). For simplicity,assume that a desired value of the delay-parameter is known in advance.In addition, other goals, such as how often an in-point is wanted, arealso known.

For the simplest case, encoding all I-frame low-delay format, thecreation of the bitstream becomes a matter of rate-control. For eachframe, there is a not-to-be-exceeded bit-count. The rate-control task isto encode each frame with the best quality possible within thebit-budget. The per-frame bit budget is computed as thetransmission-bit-rate divided by the frame-rate.

For complex GOP encoding a forward analysis of the created stream may bemade. The allocation of bits among frames must be done to assure that adecoder buffer doesn't underflow. A first constraint which may beapplied to the created stream is the defining of a splice segment as afixed GOP structure (e.g., 13 frames arranged in the following displayorder: ". . . IBBPBBPBBPBBP . . . "). This approach is straightforward athe expense of unnecessarily degraded picture quality. For example, ascene cut on the last P frame of an ". . . IBBPBBPBBPBBP . . . " GOPwould be reproduced with a very small bit budget. Unfortunately, thereis no single GOP structure that is ideal for all applications. Moreover,the loss of flexibility implied in this approach is probablyunacceptable.

A second constraint which may be applied to complex GOP encoding is theinsertion of in-points at out-points at predetermined time intervals(e.g., 2 and 0.5 seconds, respectively). This approach does not requirethe use of a specific GOP structure, therefore the encoder is free toselect frame type based upon the input pictures.

There are various rate-control issues to be resolved when switchingbetween MPEG streams or splice segments. As such, it may be necessary toconstrain the contents or size of, e.g., coded picture bitstreams. Onerate-control issue involves the amount of data transmitted to a decoderbuffer. For example, the decoder buffer will not overflow if the buffercontents (measured in bits) at any out-point is less-than-or-equal-tothe decoder buffer contents (measured in bits) measured at the mostrecent in-point. It is not necessary to know the actual number of bits,it is only necessary to ensure that the number of bits in the decoderbuffer does not increase from in-point to out-point. It must be notedthat so-called "stuffing bits" are not counted, since these bits areonly included to meet specified transmission rates and disappear fromthe buffer when the real bits are used (i.e., stuffing bits do notaccumulate in the buffer).

Another rate control issue involves the presentation time of decodedinformation frames in relation to the time the buffer receives the nextframe. For example, the decoder buffer will not underflow if, at anout-point, the time (measured in time units) to display the not-yetdisplayed buffer contents is greater than the time (measured in timeunits) for the first I-frame to be delivered to the decoder at thespecified bit rate. In other words, the next I-frame (the first frame ofthe to-stream) must be delivered to the decoder buffer before the bufferis emptied.

The above-described amount of time may be defined at the"Delay-Parameter" for the stream. The frame sizes (measured in time totransmit the frames at the operating bit rate) must be consistent withthe operating delay parameters to ensure seamless splicing. The delayparameters are the end-to-end VBV size (measured in time) and the VBVcontents (measured in time) at the beginning/end of a stream. Anadditional, globally defined value is the maximum size of physicalbuffers (in bits). This maximum size must be greater than the maximumVBV size implied by the MPEG profile and level indication criteria.Finally, the decode time stamp at an out-point of a from-stream shouldbe one frame time of the stream greater that the DTS and PTS of the lastframe of the from-stream.

As previously mentioned, it is important to distinguish between asplicing decision and the actual process of splicing. A splicingdecision is made by some human. The decision may be made in the processof generating a list of programming to be transmitted by a televisionstudio or in real time as the studio is transmitting. The splicingdecision may be made by some surrogate process, such as a preprogrammedcommand to splice a station identification announcement into the studiotransmission every day at 12:05 AM. The decision may be to splice at aparticular time in the future or immediately.

Several parameters of the streams to be spliced may be known at the timeof the decision, though these parameters may change prior to the actualsplicing operation. For example, the splicing decision is usually madewith some knowledge of the to-stream, such as the stream length, VBVdelay parameter and the like. It is possible for the from-stream to beunknown at the time of the decision (e.g., the daily message is insertedinto whichever stream is being transmitted at the time of insertion).

A decision contains the following elements. First, the operational unitwhich sources the to-stream, the operational unit which performs thesplice and the stream or segments to be spliced. Second, the time thesplice is to take place. The time may be "now," a particular time of dayor the occurrence of some logical condition. "Now" means make the nextsplice after the arrival of the splice-now message. The now decision mayarise from direct human action (e.g., button-press) or some externalcontrolling process deciding to send a splice-now message. The logicalcondition may be the occurrence of a time-code (e.g., SMPTE) in aparticular information stream, a time stamp (e.g., PTS or DTS) in avideo or audio stream, a reference time (e.g., PCR) in an informationstream or some other detectable event (e.g., an input stream PIDchanges). The logical events may be combined in a logical manner todetermine a splice time and select appropriate streams for splicing.

After a decision is made it is communicated to the appropriateoperational unit. Since there is a possibility of errors (e.g., due tothe complexity of combining multiple logical events) it is important forthe controlling entity to monitor the splicing process closely and adaptto error conditions and other conditions.

As a result of the decision, and part of making the splice work, is theprocess of assuring that the correct bitstream is applied to the correctinput of the operational unit at the correct time. This is a function ofoverall system operation, and may involve synchronization of informationflow from a server to the switcher. After receiving the splicingdecision the splicer will make the splice. The process of seamlesssplicing requires that the to-stream is entered at an in-point, thefrom-stream is exited at an out-point and, possibly, the management ofone or more buffer and timing parameters.

Illustrative Examples of Video Splicing

The above-described concepts will now be illustrated using severalexamples of MPEG bitstream splicing operations. The first example is anall I-frame low delay splicing example. A to-stream comprises either 24or 30 frames per second (fps) video streams including only I-frames. Thedelay parameter of the to-stream is equal to one frame time at theslowest frame rate (i.e., 42 mS if 24 fps). In this example each of theI-frames contains fewer bits than can be sent in one unit of displaytime (i.e., one 42 mS frame time) at the bit rate for the frame. If thebit rate is 150 Mb/s, a 30 fps frame contains no more than 5 Mb. If thebit rate is 150 Mb/s, a 24 fps frame contains no more than 6.25 Mb.

When the last bit of a from-stream having the same parameters hasentered a decoder buffer, the presentation time stamp indicating whenthe last frame is to be presented has a value 42 mS in the future. Thus,if the from-stream were at 30 Hz. (33 mS frame rate), 9 ms. after theout-point the last frame of the from stream will be taken from thedecoder buffer, and 33 mS later the first frame of the to-stream will beneeded. If the to-stream is also 30 Hz, the first frame will have beendelivered 9 mS before it is needed. If the from-stream were at 24 Hz,and the to-stream were also at 24 Hz, the to-stream frame arrives justin time. If the from-stream were at 60 Hz, (17 mS) then when the lastbit of the from-stream is delivered, the decoder buffer contains 2frames (33 mS) and the decoder will not use the first of them for 9 mS.If the bit-rates of the from and to sequences differ, as long as thebits are delivered at the rate corresponding to the sequence bit-rate,the time to load frames remains correct. To summarize, streams are codedwith a bit-count between the in-point and following out-points that iscalculated from the bit-rate and the frame-time (i.e., bit-rate *frame-time). The presentation time-stamps are set to values that allagree with the delay-parameter (i.e., first frame presenteddelay-parameter after the first bit arrives.).

The second example is a complex GOP transmission format. For purposes ofthe second example, it is assumed that stream is a 30 frame per secondvideo (and associated audio) stream having delay-parameter of 250 mS, adisplay order of ". . . IBBPBBPBBPBBPBBP . . . " and transmission orderof ". . . IPBBPBBPBBPBBPBB . . . " (where "I" represents an I-frame, "P"represents a P-frame and "B" represents a B-frame). This GOP structureincludes in-points on the I-frames and out-points on the frameimmediately preceding the I-frames. At each out-point, the PTSassociated with the last P frame is 250 mS in the future. The ratecontrol ensures that the decoder buffer doesn't underflow on theI-frame. It must contain less than 250 mS worth of bits. The valid-MPEGconstraint implies that the P-frame following the I frame also doesn'tunderflow. It is not necessary that the I-frame use all 250 mS. If theI-frame does use all 250 ms, the next P-frame must use less than 33 mS(i.e., if the I-frame used 230 ms, the next P-frame would be permitted,though not required, to use 53 mS).

The third example is a multiple output example. For purposes of thethird example it is assumed that stream is a 30 frame per second video(and associated audio) stream having the following GOP structure: ". . .IPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPPIPPPP . . . " The stream isalso assumed to have a delay-parameter of 250 mS and a transmissionbitrate of 20 Mb/s (i.e., 670 Kb per frame). If an I-frame takes 231 mSand P-frame takes 20 mS then, after 15 frames, the decoder buffercontents have subsided to a level below the level at the in-point to thestream. This may be calculated using an equation such as the following:

    (231+20N)=33*(N+1)

The above equation represents the time for bits entering, starting withan I-frame's 231 mS worth of data, followed by N 20 mS P-frames, beingequated to the time for the same N+1 frames-worth of bits to be takenfrom the buffer (N+1) frames, each taking 33 mS. In this example eachI-frame may be an in-point and all the P-frames after the fifteenthP-frame may be out-points.

The decoder buffer reacts as follows. At the splice point, the buffercontains 7 frames and a presentation time stamp indicates that the lastframe is to be presented in 250 mS. The 7 frames each use 20/33 of thebits/frame quota (i.e., each P-frame frame is 400 Kb and the buffercontains 2.8 Mb). Over the next 7 frames, the I-frame comes in,increasing the buffer contents. Since each I-frame adds 670 Kb and eachP-frame taken out removes 400 Kb the buffer contains 4.7 Mb after theseven P-frames are taken out. The I-frame is then presented, removing4.6 Mb from the buffer and, therefore, leaving 100 Kb in the buffer. Thedelay in the buffer is approximately zero. Each P-frame now adds 400 Kbin 20 mS and every 33 ms 400 Kb is used. Thus, delay in buffer increasesby 13 mS every frame time. After 15 frames, the delay stored in thebuffer has reached the delay-parameter value. At this time a splice toanother sequence may be made because the buffer is able to receive anI-frame.

Compressed Audio Splicing

The following discussion of splicing of compressed audio is limited tothe issue of splicing combined video-audio streams in theaudio-follows-video mode. The composition of streams from separatelyedited audio and video streams is not considered here.

Compressed audio is carried in frames. Each audio frame is of fixedduration and contains a fixed number of bits. Unfortunately, the audioframe size, or duration, is different from any of the video frame sizes,or durations. This means that audio frames will not align with splicepoints. Audio frames can be considered to be randomly aligned with thevideo. Therefore, when making a splice, the alignment of the audio withthe video will be different for the to-stream and the from-stream.

It is desirable to ensure alignment of audio information tocorresponding video information (i.e., "lip-sync"). That is, the audioand video must remain properly phased with respect to each other. APresentation Time Stamp (PTS) exists in each audio stream. The audio andvideo PTSs refer to the same reference to allow the requiredsynchronization. When the splice is complete, the to-stream becomes theoutput stream. It is important to note that, particularly due to audioconstraints, the process of switching may extend in time before andafter the actual switch instant.

Audio information frames in an information stream are ideally locatedwithin a limited time difference from respective video informationarrival at the end of the decoder buffer. If there is a 1/2 secondend-to-end video buffer delay, then audio packets should beapproximately 1/2 second later in a transmission stream thancorresponding (i.e., having the same presentation time stamps) videopackets. If this assumption is correct, then the switching operationalunit must save audio information from the from stream for this 1/2second after the video switch. The source stream must continue for 1/2second after the splicer has switched to another stream. It is alsoamusing to contemplate rapid switching among several streams.

Three approaches to splicing audio will now be described. First, theoverlapped audio packets may simply be broken. This is not the mostdesirable approach because it relies upon the CRC to prevent the use ofpartial packets. If the CRC fails one time in 64 K packets, at about 30packets/second, every few thousand seconds there is a potentialundetected error. When a broken-packet CRC fails, it fails every timethe packet is used. This means that a failure that produces a click mayend up reproduced every time the same splice is made. This requires theuse of a garbage-collecting process to remove broken audio frames.

A second approach to splicing audio is to include "unfinished"from-stream audio frames (i.e., those overlapping a video splice) in theresultant stream. The to-stream audio frames are then retimed such thatthey butt up against the "unfinished" from-stream audio frame. Thistechnique assures continuous audio at the expense of continuousinspection, buffering and adjustment of audio frames and packets. Thefirst complete to-stream audio frame is selected as the first audioframe to play because the to-stream frame which has already started islikely to be delayed too much to match the end of the "unfinished"from-frame. This technique also causes a slightly distorted lip-sync.

A third approach to splicing audio is to maintain alignment of audiowith its corresponding video, that is, to leave a gap between the end ofthe from-stream audio frame and the beginning of the to-stream audioframe. This approach advantageously relies on the MPEG decoderspecification which requires that audio frame gaps are muted. In thisapproach the audio presentation-time-stamps are adjusted by the sameadjustment amount applied to the video frames. In this manner lip syncis maintained. The third approach is especially useful when many splices(e.g., the creation of a sequence from a succession of short splicesegments) may produce audio artifacts due to the muting.

FIG. 5 depicts a plurality of splicing scenarios involving audioalignment which illustrate aspects of audio-video splicing, assuming theabove cited third approach is used to maintain alignment of audio withits corresponding video.

FIG. 5A depicts the simple splicing case where both audio streams alignwith their corresponding video. The splicer delays both to-streams andsimply switches at the splice point.

FIG. 5B depicts the splicing case where the from-stream video andfrom-stream audio are aligned, but the to-stream video and to-streamaudio are not aligned. When the splice is made, a partial to-streamaudio frame is discarded. The next complete to-audio frame is passed tothe output with appropriate delay.

FIG. 5C depicts the typical splicing case where both audio streams donot align with the corresponding video streams. It can be seen that afrom-stream audio frame has already begun before the splice point. Thisaudio-frame is buffered and transferred to the output. It doesn't enduntil some fraction of a frame time after the splice. The to-streamaudio frame that spans the splice-point cannot be used. The nextto-stream audio frame also cannot be used. It begins too early, andwould overlap the last from-stream audio frame. The first to-streamaudio frame that appears in the output stream begins D time units afterthe splice-point. This delay may be as much as two audio frames.

In the above example (FIG. 5C) the lip-sync is preserved, but as much as32 mS of from-stream audio overlaps the to-stream video. Also, the firstto-stream audio begins as late as 64 mS after to-stream video begins.Finally, the splicer performing the splicing operation must buffer awhole audio frame in each work buffer.

Ideally, every audio frame includes a PTS. It is possible that someequipment manufacturers only include a PTS every, e.g., third audioframe. In this case, or the case where there is no audio PTS, a splicingoperation may be performed after calculating a virtual time-stamp. Thevirtual time stamp is derived from the approximate real-time delay ofaudio-frames from video reference time-stamps. The virtualaudio-time-stamp is then incremented by the (known) audio-frame durationon successive audio-frame starts. This process may be implemented as abackup process to ensure that non-time stamped audio streams to not getinto a studio where time stamps are crucial to the operation of thestudio.

Auxiliary Data Splicing

Auxiliary data is included in many MPEG streams. This data is usuallypresent as contiguous, unbreakable streams of unknown length. Bycontrast, a compressed audio stream is relatively well-behaved andpredictable. The auxiliary data stream may be associated with acorresponding video stream having a delay that is not now specified.

There are several methods for handling auxiliary data in a splicingoperation, including: 1) ignore it and splice the auxiliary data at thesame time as the video data; 2) insert auxiliary data through a separatepath to, e.g., a play-to-air switcher (this data may comprise a programguide or other consumer-related information); and 3) define a set ofsegmentation markers for auxiliary data and rely upon these markers inswitchers to keep segmentation correct (this requires knowledge aboutthe content of auxiliary data streams and the lengths of auxiliary datasegments within those streams). The auxiliary data may also be switchedwith or without a delay, and the delay may be a parameter passed to theswitcher by a decision making controller. The auxiliary data may beinput to the splicer via an auxiliary input. Input arriving on theauxiliary-data input may be buffered and inserted into the output streamon a space-available basis as a replacement for null packets. In thiscase it becomes some other system unit's responsibility to align suchdata within streams, and to provide channel capacity for insertedaux-data by, e.g., reducing a video stream data rate.

Although various embodiments which incorporate the teachings of thepresent invention have been shown and described in detail herein, thoseskilled in the art can readily devise many other varied embodiments thatstill incorporate these teachings.

What is claimed is:
 1. A method for splicing bitstreams, said bitstreamscomprising a plurality of information segments representative of asequence of information frames, said method comprising the stepsof:monitoring a first bitstream to detect an exit indicium, said firstbitstream being coupled to an output and including at least one of saidexit indicium, said exit indicium indicative of an appropriate lastinformation segment of a splicing segment; providing a second bitstream,said second bitstream including at least one entrance indicium, saidentrance indicium indicative of an appropriate first information segmentof a splicing segment; and coupling said second bitstream to said outputin response to a splice decision signal, a detection of said exitindicium in said first bitstream and a detection of said entranceindicium in said second bitstream, said step of coupling comprising thesteps of:decoupling said first bitstream from said output at a detectedexit indicium; coupling said second bitstream to said output at adetected entrance indicium; and transmitting a splice acknowledgmentsignal.
 2. The method of claim 1, wherein:said splice decision signalcomprises at least one of a desired temporal parameter, a desired splicequality parameter and a priority parameter; and said spliceacknowledgment signal comprises at least one of an actual temporalparameter and an actual splice quality parameter.
 3. The method of claim2, wherein:said desired temporal parameter is indicative of a desiredtime to perform said coupling step; said desired splice qualityparameter is indicative of a minimum splice quality level, said minimumsplice quality level being less than or equal to a seamless splicequality level; said actual temporal parameter is indicative of a timesaid coupling step was performed; and said actual splice qualityparameter is indicative of a quality level of a bitstream producedduring said coupling step.
 4. A method for splicing bitstreams, saidbitstreams comprising a plurality of information segments representativeof a sequence of information frames, said method comprising the stepsof:monitoring a first bitstream to detect an exit indicium, said firstbitstream being coupled to an output and including at least one of saidexit indicium, said exit indicium indicative of an appropriate lastinformation segment of a splicing segment; providing a second bitstream,said second bitstream including at least one entrance indicium, saidentrance indicium indicative of an appropriate first information segmentof a splicing segment; and coupling said second bitstream to said outputin response to a control signal, a detection of said exit indicium insaid first bitstream, a detection of said entrance indicium in saidsecond bitstream, said step of coupling comprises the steps of:comparinga delay parameter associated with said first bitstream to a delayparameter associated with said second bitstream; and coupling saidsecond bitstream to said output in response to said delay parametersbeing substantially equal.
 5. The method of claim 4 wherein said controlsignal comprises a splice decision signal, and said step of couplingfurther comprises the steps of:decoupling said first bitstream from saidoutput at a detected exit indicium; coupling said second bitstream tosaid output at a detected entrance indicium; and transmitting a spliceacknowledgment signal.
 6. The method of claim 5 further comprising thestep of:storing, in response to a detected entrance indicium in saidsecond bitstream, said appropriate first information segment and aplurality of successive information segments associated with saiddetected entrance indicium; and wherein: said splice decision signalincludes at least a priority parameter, and said step of couplingfurther comprises the steps of: increasing or decreasing a delayparameter associated with said stored information stream in response toa predetermined priority parameter.
 7. A method for splicing a firstinformation stream into a second information stream to produce an outputinformation stream, each of said information streams comprising aplurality of information segments representative of a sequence ofinformation frames, said method comprising the steps of:coupling saidsecond information stream to an output to produce said outputinformation stream; detecting an intra-coded information frame in saidfirst information stream, said first information stream including atleast one of said intra-coded information frame, said intra-codedinformation frame indicative of an appropriate first information segmentof a splicing segment; storing, in response to said intra-codedinformation frame, said first information segment in a first portion ofa memory, said information segments representative of a sequence ofinformation frames succeeding said intra-coded information frame beingstored in a second portion of memory, and a plurality of successiveinformation segments, said step of storing comprises the stepsof:storing information segments representative of said intra-codedinformation frame in a first portion of a memory; storing informationsegments representative of a sequence of information frames succeedingsaid intra-coded information frame in a second portion of memory;repeating said step of storing each time an intra-coded informationframe is detected in said first information stream; detecting an exitindicium in said second information stream, said second informationstream including at least one of said exit indicium, said exit indiciumindicative of an appropriate last information segment of a splicingsegment; retrieving, in response to said exit indicium, said storedinformation segments; and coupling said retrieved information segmentsto said output to produce said output information stream.
 8. Apparatusfor splicing information streams, said apparatus comprising:a bufferreceiving a first information stream and producing a bufferedinformation stream; a bitstream examiner, coupled to said buffer,receiving said first information stream and responsively causing saidbuffer to position an entrance point of said buffered information streamat an output of said buffer; a switch, for coupling either said bufferedinformation stream or a second information stream to an output; and aswitch controller, for transmitting a splice acknowledgement signal,coupled to said switch and receiving said second information stream anda splice decision signal, for monitoring said second information streamand, in response to said splice decision signal and the detection of anexit point in said second information stream, causing said switch tocouple said buffered information stream to an output.
 9. The apparatusof claim 8, wherein:said switch controller compares a delay parameterassociated with said buffered information stream to a delay parameterassociated said second information stream; and said switch controller,in response to said delay parameters being substantially equal, causessaid switch to couple said buffered information stream to said output.10. The apparatus of claim 9, wherein said switch control signalcomprises a splice decision signal including at least a priorityparameter, and said switch controller transmits a splice acknowledgmentsignal.
 11. The apparatus of claim 9, wherein said switch controller iscoupled to said bitstream examiner, and wherein:said switch controller,in response to a predetermined priority parameter of said switch controlsignal, causes information to be added or deleted from said buffer suchthat said delay parameter of said buffered information stream is,respectively, increased or decreased.
 12. Apparatus for splicinginformation streams, said apparatus comprising:a buffer receiving afirst information stream and producing a buffered information stream; abitstream examiner, coupled to said buffer, receiving said firstinformation stream and responsively causing said buffer to position anentrance point of said buffered information stream at an output of saidbuffer, said buffer being flushed by said bitstream examiner each timean entrance point is detected in said first information stream; aswitch, for coupling either said buffered information stream or a secondinformation stream to an output; and a switch controller, fortransmitting a splice acknowledgement signal, said spliceacknowledgement signal comprising at least one of an actual temporalparameter and an actual splice quality parameter, coupled to said switchand receiving said second information stream and a splice decisionsignal, for monitoring said second information stream and comprising atleast one of a desired temporal parameter, a desired splice qualityparameter and a priority parameter, and the detection of an exit pointin said second information stream, causing said switch to couple saidbuffered information stream to an output.
 13. The apparatus of claim 12,wherein:said desired temporal parameter is indicative of a desired timeto couple said buffered information stream to said output; said desiredsplice quality parameter is indicative of a minimum splice qualitylevel, said minimum splice quality level being less than or equal to aseamless splice quality level; said actual temporal parameter indicativeof a time said buffered information stream was coupled to said output;and said actual splice quality parameter is indicative of a qualitylevel said output information stream.